1. Field of the Invention
The present invention relates to an active matrix substrate including switching devices such as thin film transistors (hereinafter, referred to as "TFTs") and a liquid crystal display device (hereinafter, referred to as an "LCD device") including the active matrix substrate.
2. Description of the Related Art
An LCD device includes an active matrix substrate and a counter substrate between which is interpose a liquid crystal material acting as a display medium therebetween. FIG. 5 shows an exemplary structure of an active matrix substrate including switching devices.
The active matrix substrate shown in FIG. 5 includes a plurality of TFTs 23 acting as switching devices arranged in a matrix and a plurality of picture element capacitors 22 also arranged in a matrix. A plurality of gate lines 24 acting as scanning lines for controlling the TFTs 23 are connected to gate electrodes of the respective TFTs 23. The TFTs 23 are each driven by a signal input to the respective gate lines 24. A plurality of source lines 26 acting as signal lines for supplying data signals to the TFTs 23 are connected to source electrodes of the respective TFTs 23. Video signals or other signals are input to the source lines 26. The gate lines 24 and the source lines 26 are arranged to cross each other. A drain electrode of each TFT 23 is connected to a picture element electrode and one of two terminals of the respective picture element capacitor 22. The other terminal of each picture element capacitor 22 is connected to a picture element line 25. The picture element line 25 is to be connected to a counter electrode in the counter substrate when the active matrix substrate is combined with the counter substrate to produce an LCD device.
FIG. 6 is a partial plan view of a conventional LCD device including such an active matrix substrate. FIG. 7A is a cross sectional view of the LCD device along line E-E' in FIG. 6, and FIG. 7B is a cross sectional view of the LCD device along line F-F' in FIG. 6.
As shown in FIG. 6, the active matrix substrate includes a plurality of gate lines 20 and a plurality of source lines 70 which cross each other. A picture element area is defined by the two adjacent gate lines 20 and two adjacent source lines 70. FIGS. 6, 7A and 7B each show one picture element area of the active matrix substrate.
As shown in FIG. 7B, the active matrix substrate includes a light-transmissive insulation plate 1, a gate electrode 2 provided on the plate 1, and a gate insulation layer 3 provided on the plate 1 so as to cover the gate electrode 2. A semiconductor layer 4 is provided on the gate insulation layer 3 above the gate electrode 2, and a channel protection layer 5 is provided on a central area of the semiconductor layer 4. A source electrode 6a and a drain electrode 6b both formed of n.sup.+ -Si are provided on the semiconductor layer 4 so as to cover the channel protection layer 5. The source electrode 6a and the drain electrode 6b are separated from each other on the channel protection layer 5. A source line 70 including an ITO (indium-tin-oxide) film 7a and a metal layer 8a provided on the ITO film 7a is provided on the gate insulation layer 3 so as to cover the source electrode 6a. A connecting electrode 7' formed of an ITO film 7b is provided on the drain electrode 6b, and a metal layer 8b is provided on the connecting electrode 7'. The connecting electrode 7' is extended on the gate insulation layer 3 for connecting the drain electrode 6b and a picture element electrode 11.
As shown in FIGS. 6 and 7A, the connecting electrode 7' is extended to cover a picture element storage capacitor signal line 19. An area where the connecting electrode 7', the gate insulation layer 3 and the picture element storage capacitor signal line 19 overlap acts as a storage capacitor.
An interlayer insulation layer 9 is provided on the gate insulation layer 3 so as to cover the gate line 20 (FIG. 6), the source line 70, and a TFT 23 including the gate electrode 2, the source electrode 6a and the drain electrode 6b. The picture element electrode 11 is provided on the interlayer insulation layer 9. The picture element electrode 11 is connected to the drain electrode 6b via the connecting electrode 7' at the bottom of a contact hole 10 formed so as to run through the interlayer insulation layer 9 in the thickness direction. In FIG. 7B, reference numeral 2a denotes an anodized film formed on the surface of the gate electrode 2. In FIG. 7A, reference numeral 19a denotes an anodized film formed on the surface of the picture element storage capacitor signal line 19.
In the active matrix substrate having the above-described structure, the interlayer insulation layer 9 is provided between the gate line 20/source line 70 and the picture element electrode 11. Accordingly, end parts of the picture element electrode 11 can be overlapped with the gate line 20 and the source line 70. Due to such a structure, the numerical aperture is improved, and an electric field generated by the gate line 20 and the source line 70 is shielded, thus preventing the defective orientation of liquid crystal molecules.
In the above-described structure, the picture element electrode 11 is connected to the drain electrode 6b through the contact hole 10 formed in the interlayer insulation layer 9, and thus the picture element electrode 11 has a stepped portion having a depth corresponding to the thickness of the interlayer insulation layer 9. The orientation of the liquid crystal molecules is disturbed over the stepped portion, possibly causing the light to be transmitted where the light should not be transmitted.
Conventionally, in order to solve this problem, areas corresponding to the contact holes are masked by, for example, using light-shielding materials for the metal electrodes, the gate lines, the source lines, the picture element storage capacitor signal lines or by a black matrix of a color filter. Accordingly, the opening of each picture element area is decreased, which prevents improvement of the numerical aperture.
In the cage of a reflection-type LCD device, the stepped portion of the picture element electrode changes the surface state of the picture element electrode and thus deteriorates a reflection characteristic. Conventionally, in order to solve this problem, areas corresponding to the contact holes are masked by a black matrix of a color filter. Accordingly, the opening of each picture element area is decreased, which prevents improvement of the numerical aperture.